The aim of the work is to study the nature of apparent free radicals generated inherently during the activity of certain flavin oxidoreductases in animal tissues, and to determine the type of alterations in membrane structure and function which occur as a result of their interaction with the radicals. In addition, we will attempt to evaluate the possible role of this phenomenon in certain degradative processes, such as aging and in pathological states, which are believed to be due to exposure of animals to certain organic compounds, the metabolism of which almost certainly involves free radical intermediates. We will also evaluate the possibility that naturally occurring free radical scavenging agents may be effective in protecting cell structures from alterations caused by free radicals generated during the activity of some enzymes but not by others because of a non-uniform distribution of scavengers in membranous organelles.